Background: Hemifacial spasm (HFS) is a chronic and disabling movement disorder commonly affecting older adults. Although botulinum toxin (BoNT) is an established treatment, standardized and multidimensional assessments of severity remain limited. Aim: This study evaluates the therapeutic efficacy of the HFS Severity Score (HFSS), a newly developed scale incorporating five symptom domains, and compares its responsiveness with that of the Jankovic Rating Scale (JRS). Methods: This retrospective study included 18 patients aged 65 years or older who were treated with 10 units of BoNT. HFSS and JRS were assessed at baseline and 4 weeks post-injection by two blinded neurosurgeons. Inter-rater reliability (intraclass correlation coefficient [ICC]) and internal consistency (Cronbach’s α) were calculated. Pre–post differences were evaluated using the Wilcoxon signed-rank test. Convergent validity was examined using Spearman correlation between baseline HFSS and JRS. Responsiveness was assessed using change–score correlations and standardized response means (SRMs). Scatter plots were generated to visualize relationships between the scales. Results: Mean HFSS improved significantly from 9.39 to 3.72 (p<0.001), with all subdomains showing improvement. Baseline HFSS and JRS demonstrated a strong correlation (ρ = 0.82, p<0.001), while ΔHFSS and ΔJRS showed a weaker association (ρ = 0.32), reflecting a floor effect in post-treatment JRS scores. SRM scores indicated high responsiveness for HFSS (1.73) and JRS (1.54), while the ICC (0.91) and Cronbach’s α (0.85) confirmed excellent reliability. No adverse events were observed. Conclusion: BoNT treatment is effective and well-tolerated in older adults with HFS. HFSS is a reliable, multidimensional, and highly responsive tool that complements existing scales and enhances clinical assessment. Relevance for Patients: Older adults with HFS may achieve symptom relief with BoNT. HFSS provides a comprehensive metric that can enhance personalized care and reduce reliance on medication.